Counteranion and Solvent Assistance in Ruthenium-Mediated Alkyne to Vinylidene Isomerizations

Abstract

The complex [Cp*RuCl((i)Pr2PNHPy)] (1) reacts with 1-alkynes HC≡CR (R = COOMe, C6H4CF3) in dichloromethane furnishing the corresponding vinylidene complexes [Cp*Ru═C═CHR((i)Pr2PNHPy)]Cl (R = COOMe (2a-Cl), C6H4CF3 (2b-Cl)), whereas reaction of 1 with NaBPh4 in MeOH followed by addition of HC≡CR (R = COOMe, C6H4CF3) yields the metastable π-alkyne complexes [Cp*Ru(η(2)-HC≡CR)((i)Pr2PNHPy)][BPh4] (R = COOMe (3a-BPh4), C6H4CF3 (3b-BPh4)). The transformation of 3a-BPh4/3b-BPh4 into their respective vinylidene isomers in dichloromethane is very slow and requires hours to its completion. However, this process is accelerated by addition of LiCl in methanol solution. Reaction of 1 with HC≡CR (R = COOMe, C6H4CF3) in MeOH goes through the intermediacy of the π-alkyne complexes [Cp*Ru(η(2)-HC≡CR)((i)Pr2PNHPy)]Cl (R = COOMe (3a-Cl), C6H4CF3 (3b-Cl)), which rearrange to vinylidenes in minutes, i.e., much faster than their counterparts containing the [BPh4](-) anion. The kinetics of these isomerizations has been studied in solution by NMR. With the help of DFT studies, these observations have been interpreted in terms of chloride- and methanol-assisted hydrogen migrations. Calculations suggest participation of a hydrido-alkynyl intermediate in the process, in which the hydrogen atom can be transferred from the metal to the β-carbon by means of species with weak basic character acting as proton shuttles.

Molybdenum Trihydride Complexes: Computational Determinations of Hydrogen Positions and Rearrangement Mechanisms

“…These values correspond well with a recent theoretical report for a similar reaction in Ru tryhidride complexes. 34 Even for the Cp t Bu 3 molecule, the Gibbs free energy diagram (Figure 7) shows that TS R1 has a lower energy than the H 2 complex. The energy of TS R1 is only above that of the H 2 complex (TS R1 = 2.65 kcal/ mol, H 2 complex = 2.64 kcal/mol) for the electronic energy.…”

Directionality of Dihydrogen Bonds: The Role of Transition Metal Atoms

“…Participation of the metal atom in hydrogen bonding at a hydride site was shown to assist direct proton transfer to W and Os in the case of [Cp*WH 3 (dppe)] [21] (Cp* = h 5 -C 5 Me 5 ) and [Cp*OsH(dppe)] [22] (dppe = k 2 -Ph 2 PCH 2 CH 2 PPh 2 ). [21,26] Interestingly short metal-proton distance and significant Mo···HF overlap population were found in one of the first computational works on DHB [27] but were barely discussed. [24] For the [Cp*MH(dppe)] series, passing from ruthenium to osmium changes the reaction mechanism: proton transfer yields dihydrogen complex for Ru but cis-dihydride for Os.…”

Copper-Catalyzed Regioselective Synthesis of (E)-β-Fluorovinyl Sulfones

“…These results suggested that the methanol plays an important role in the stereoselectivity and the regeneration of the catalytically active copper species. The coordinated methanol has also been observed to exert important effects in other reactions catalyzed by this copper complex [19], as well as in other metal-mediated transformations of alkynes [20].…”